Thanks for sharing your experience.
China has the biggest electric bike market on Earth, and has some good components at much lower prices than a Bosch for your project.
I have been thinking about strapping a kilowatt Bafang (8Fun) motor onto a bike (http://lunacycle.com/mid-drive-kits/), but I will play with the off the shelf ebike for a while first. You can strap a big 48v or 52v battery on it, and overdrive it way beyond 1,000 watts if you want to hot rod.
No plates, no license, no insurance.
There is an Australian company making expensive ebikes that can do 50 mph! (http://www.stealthelectricbikes.com/stealth-h-52-hurricane/) So much quieter than a BMX bike.
I put a 750 watt “Golden” brand “brushless hub motor front wheel that I purchased through ebay on a “comfort” bike. The advantages over an off the shelf bike is that it used a motorcycle style accelerator grip and would do about 25mph on the flat without pedaling. I am not sure but I believe that most electric bikes that you can purchase are limited to about 15mph. It cost much less than an off the shelf bike.
I still have an electric scooter with a geared hub motor that has brushes. The scooter has a lower wattage motor but has much more torque than the bicycle did.
The controller for the brushless hub motor creates a rotating magnetic field that drags a bunch of permanent magnets mounted in the hub around with it. Unlike a motorcycle or car if you give it full throttle it actually does not give you as much torque and acceleration as when you advance the throttle so that the magnetic field is rotating just slightly faster than you are traveling. It takes a little getting used to. But the motors are efficient, and have a good top speed.
I am one of these people who take almost everything apart and the brushless motor is a simpler design than the geared hub motors. All it has in it are bearings stationary coils and rotating permanent magnets. I appreciate greatly the simplicity of the device. They can also be easily used as generators and controllers are available that use the hub motors for regenerative braking.
One has to be a little skeptical of the specifications on eBay, but you can still put together a bike that is much faster and more powerful than one you can buy, and cheaper also. It I not very difficult and complete kits are available. I would encourage you to put something together yourself... depending on how mechanically inclined you are.
Looking at the Golden Motors website, I can see that the motors looked refined since I purchased the kit that I installed. The hub motor kit that I purchased was a good quality item. If you look toward the bottom at the ebike DIY section you will see that they have an incredible amount of options for a reasonable price.
I don't know about the lunacycle kits you linked to. A hub motor based kit is much easier to install can be found to fit almost any bicycle. I also have put together a gasoline power bicycle which is what the lunacycle kits remind me of. It was more difficult to put together, and I had to find just the right bicycle to put it in. They simply will not work in any bike. But it was a kick in the pants. Mine was an 80cc two stroke bolted to inside of the frame and it was amazing to have impressive power on what was a basically a motorcycle that weighed only about 70 pounds. It had very little torque when starting out, but it would do about 40mph and when you hit the “power band” it was almost scary.
They used to have a bunch of forums devoted to both electric and gasoline powered bikes. There are lots of pictures and people willing to share what they have learned.